The screw is the metal artifact. The head with the metal obstruction is a model for testing algorithms for removing the effect of metal objects such as metal dental posts from x ray images.

The model is made from a soft substance, here called gelly, for some reason. Gelly is an obsolete name for jelly.

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MAR is designed to reveal anatomic details obscured by metal artifacts, helping clinicians utilize CT scans, diagnose disease and contour targets with greater confidence.
http://www3.gehealthcare.com/en/products/categories/computed...

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O -MAR (Metal Artifact Reduction for Orthopedic Implants) is a commercial product available from Philips Healthcare which implements a robust and efficient algorithm to mitigate artifacts caused by metal objects in CT images.
http://clinical.netforum.healthcare.philips.com/us_en/Explor...